Galls on trees

As the leaves on your deciduous trees and shrubs begin to change colour and fall to the ground your eye might be taken by one of the many unusual growths which might be found on their branches or leaves.

Some of these growths are caused by gall wasps. They are more common on native trees and shrubs than on cultivated varieties because they are part of a natural long-term ecosystem in which other species have developed alongside them. The single best tree on which to search for galls is the oak.

Gall wasps and oak trees

The many different galls found on an oak tree are home to larvae of some thirty species of gall wasp. The female wasps lay their eggs on the oak tree, in its buds, leaves, flowers or roots in spring and summer. 

At the same time they inject a little growth hormone creating a strange pattern of growth in the tree, so it is the tree rather than the gall wasp which creates the gall shape. The eggs hatch into grubs which then develop inside the gall. Unfortunately for the gall wasp there are other wasp species that feed on their larvae but as a means of defence the galls often contain high concentrations of tannin, which is a distasteful substance helping to deter attackers.

Marble galls are one of the easiest types to identify; they have the size and shape of a marble and occur on thin branches of the oak tree. In October it should be possible to see the small, round exit holes of the wasp larvae. If you find one that has an enlarged exit hole then it may have been attacked by a great spotted woodpecker which has probably eaten the gall wasp larva.

On the underside of leaves you may find the small flattened spangle galls which, in exceptional cases, can number up to one hundred on each leaf. The grubs inside the spangle galls over-winter inside them before emerging in the spring to cause yet another type of gall, the currant gall, on the oak’s catkins.

One of the prettiest galls is also found under the oak’s leaf. The silk button gall, looks exactly as its name suggests with a rolled edge and central indentation being golden brown and silky in appearance. The gall wasp to which this belongs has two generations each year, one being sexual the other being agamic (all female without the need for a male to reproduce). It is the agamic population which causes the silk button galls on the underside of oak leaves, the sexual population creates blister galls which are found on oak leaves earlier in the year.

Other galls include the cherry gall which is found on the leaf and the artichoke gall which forms on the branches of an oak tree having formed from an egg laid inside a leaf bud. Both of these bear a striking resemblance to the subjects after which they were named.

Knopper gall

Knopper gall

One gall that has attracted a great deal of attention since it was first identified in southern Britain in the 1960’s is the knopper gall. It was feared that the knopper gall wasp might cause the widespread destruction of oak woodlands but their effect has not proved so devastating. The damage is caused by the larvae inside the galls; these eat away at the acorn on which this type of gall grows. Part of the life cycle of the knopper gall wasp relies upon the Turkey oak, a variety of oak introduced from Turkey in the 19th century, so the spread of the knopper gall wasp is restricted by the need to live near both types of tree.

There are many more types of gall on oak trees but not all galls are found on oaks. One of the most colourful is found on wild roses. The robin’s pincushion, or bedeguar gall, was named after the robin because of its bright red colour. Unlike the other galls mentioned here, this gall is home to a colony of gall wasp larvae each living in its own chamber within the gall.

Robin's pincushion on dog rose